1. Field
The present disclosure relates generally to communication systems, and more particularly, to cancelling transmit out of band emissions using a non-linear adaptive scheme.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communications with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA), 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, where NS≧min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system may support time division duplex (TDD) and/or frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the base station to extract transmit beamforming gain on the forward link when the multiple antennas are available at the base station. In an FDD system, forward and reverse link transmissions are on different frequency regions.
As wireless technology advances the use of wideband and multi-carrier signals in the 4G and 3G systems has evolved and has resulted in less separation between the transmit and receive frequencies. Adequate spacing between the transmit and receive frequencies is required for frequency division duplex (FDD) systems to operate correctly, especially at sensitivity. When there is adequate separation between the transmit and receive frequencies, transmission of out of band (OOB) emissions that fall within the frequency band of the receive band are weak enough that those emissions are not a significant source of interference. This is not the case in a number of frequency band classes for 3G and 4G FDD system configurations.
This lack of separation can result in desensitization of the receiver chain by several dBs, which in turn can cause significant performance degradation and possible reduced cell coverage. The main component of the transmit out of band emission is represented by spectral regrowth that is cause by the intermodulation products excited by the power amplifier non-linearities. It is the distortion products in the transmit frequency band that are responsible for the spectral regrowth in the receive band due to reduced transmit and receive separation and limited duplex rejection. This effect is more severe in 4G systems, including Long Term Evolution (LTE) and High Speed Uplink Packet Access (HSUPA), where the duplexer separation can be limited.
There is a need in the art for mitigating the problem of transmit out of band emissions using a non-linear adaptive scheme to cancel the out of band emissions falling in the receiver band.